In some tree handling equipment, a work unit depends from the free outer end of a boom and is connected thereto by a link that pivots about two axis perpendicular to one another. These link mechanisms normally permit the work unit to pivot freely on the two axes. The term tree handling equipment herein is used to include boom mounted grapples for handling trees, boom mounted processing heads, grapple type skidders and the like. It will be obvious from the description to follow hereinafter, the swing damper of the present invention can have other enviromental uses not related to tree handling equipment.
The main purpose of the two axis link, in for example a grapple depending from a boom, is to allow the grapple to freely position itself during the grappling maneuver. This, from use, has been found to produce the most efficient result. In the case of a processing head suspended from a boom, the link permits free fall of the tree in processing heads where the felling function is also included. During the processing operation, the felling head is free to react to the changing location of the trees centre of gravity, following the change in tilt of the tree as the tree is propelled endwise in a generally horizontal attitude that changes during processing. The free pivotal movement of the processing head avoids prying or bending action which might damage the head or have an adverse affect on the feed function.
There are, however, problems associated with freely pivoted work units on a boom and particularly when the boom and/or boom mounted vehicle is being maneuvered. For example, in the case of a boom mounted grapple, maneuvering of the boom can cause wild swinging action of the work unit which inhibits or slows down productivity. In the case of a grapple type skidder, the grapple has a tendency to swing wildly on its pivots during the travel phase of the vehicle when the grapple is empty. This not only generates substantial noise, but also causes destructive impacts on the vehicle itself and/or grapple.
As indicted in the foregoing, the pivotal connections between the work tool and the boom, particularly on tree handling and tree processing equipment, have been nothing more than pin connections allowing free pivotal rotation which has draw back of allowing the work tool to oscillate wildly during rapid movements of the boom required for operating efficiency in the field. It is therefore desirable to have some means to damp or restrain movement of the work unit. Known methods of controlling the swing movement involve the use of mechanical friction devices which may be variously adjustable by, for example, tightening a nut on the pivot pin mounting or by grease injections. Another known method involves the use of a control cylinder that actuates brake shoes. All require frequent adjustment to compensate for wear and frequent replacement of worn components. Because of the wear, the swing dampening effect varies widely and the devices have generally proven unsatisfactory.